Research is being conducted with the long-term aim of developing insight into the health effects and biologic role of free radicals. Several environmental insults are known to cause biological damage by free radical reactions. For example, air-borne oxidants such as ozone and nitrogen oxides produce free radicals in cells. Ionizing radiation and ultraviolet light also are agents which act by free radical mechanisms. In this proposal we suggest detailed study of several radicals which play key roles in these processes. The hydrogen atom is one of the more reactive and destructive species produced when cells are irradiated. We propose to continue our study of the reactions of the hydrogen atom with various organic and biological compounds. The hydroxyl radical also is produced in the radiation of cells and, in addition, plays an important role in oxidative damage caused by pollutants. We here propose the initiation of a study of the chemistry and biochemistry of the hydroxyl radical. Sulfur compounds and especially thiols are very efficient scavengers of free radicals and occur widely in living systems and, therefore, are involved in most radical processes in cells. For example, thiols (cysteine, cysteamine, penicillamine, AET) protect against ozone, oxygen high pressure, and radiation. Thiyl radicals are produced from thiols whenever thiols act as oxidation or radiation protectants. Some of the basic chemistry of thiols and thiyl radicals has not been studied. We here propose three projects: hydrogen abstraction by thiyl radicals; photochemistry of thiols and related sulfur compounds; and the use of tritiated thiols as labels of free radical damage to proteins and other biopolymer molecules.